I once bought a blue gradient filter. It rotated, so I thought I'd use it to add blue to any skyline. The "blue-grad" worked okay but added a blue tint to everything (Clouds, trees, airplanes) in the sky. I held on to it though.
Now I use it in a new way: to balance different light on a wall. Inside my home I had sunlight coming through a window to my right, and a table lamp on my left. Both gave off light that spilled onto a white wall. To my camcorder, one side of my video was blue and the other was orange. I rotated the blue filter to make everything blu-ish, and re-whitebalanced. Ta-da!
-andrew